Multi-stage formation of thin-films for photovoltaic devices

ABSTRACT

A method is provided for producing a film of compound material. The method includes providing a substrate and depositing a film on the substrate. The deposited film has a first chemical composition that includes at least one first chemical element and at least one second chemical element. At least one residual chemical reaction is induced in the deposited film using a source containing at least one second chemical element to thereby increase the content of at least one second chemical element in the deposited film so that the deposited film has a second chemical composition. The content of at least one second element in the second chemical composition is larger than the content of at least one second element in the first chemical composition.

FIELD

The present invention relates to a method for forming a compoundmaterial thin film, such as a semiconductor thin-film suitable for usein photovoltaic solar cells and other devices.

RELATED ART

One of the major contributors to current worldwide generation ofrenewable energy is the solar energy produced via a photovoltaic (PV)effect. PV-based renewable-energy sources generate energy, in the formof electricity, by harnessing electromagnetic radiation, such assunlight. PV applications are not limited to any particular area of theworld and/or any given sector of the economy. In remote regions of theworld, for example, an off-grid installation of the PV source providesthe only available source of electricity. In highly populated and/oreconomically developed regions, the PV source may, for example, sourceelectricity to an electrical grid to supplement and/or reduce the amountof conventional energy distributed from the electrical grid. A PV sourceis in general any electric system containing a PV device, such as a PVcell or a PV module.

PV devices are frequently used to convert optical energy into electricalenergy. Typically, a photovoltaic device is made of one or twosemiconductors with p-doped and n-doped regions. The commercializationof PV devices depends on technological advances that lead to higherefficiencies and lower cost of such devices. The cost of electricity canbe significantly reduced by using PV devices constructed from compoundthin film semiconductors, such as copper indium gallium selenide (CIGS).

Several techniques have been developed for producing thin-film PVmaterials. Thin films of alloys based on amorphous silicon can beproduced using chemical vapor deposition (CVD). CdTe films may bemanufactured in a number of different ways, which includeelectro-deposition and vapor transport deposition. The CVD process isrelatively expensive and not suitable for compound semiconductors, suchas CIGS. Less expensive techniques developed for CdTe are also notapplicable for other thin-film PV materials. CIGS films are mainly madeusing either an elemental co-evaporation process or a two-stage process,in which sputtering of a precursor film is followed by its selenization.The two-stage process is relatively easier to scale up in volume incomparison with the co-evaporation process, but it has drawbacks, suchas loss of adhesion between the absorber layer and the back contactlayer during selenization.

SUMMARY

In accordance with one aspect of the invention, a method is provided forproducing a film of compound material. The method includes providing asubstrate and depositing a film on the substrate. The deposited film hasa first chemical composition that includes at least one first chemicalelement and at least one second chemical element. At least one residualchemical reaction is induced in the deposited film using a sourcecontaining at least one second chemical element to thereby increase thecontent of at least one second chemical element in the deposited film sothat the deposited film has a second chemical composition. The contentof at least one second element in the second chemical composition islarger than the content of at least one second element in the firstchemical composition.

In accordance with another aspect of the invention, a method is providedfor producing a thin-film solar cell that includes depositing a firstcontact layer on a substrate and producing an absorber layer over thefirst contact layer. A window layer and a second contact layer aredeposited over the absorber layer. The absorber layer is produced by aprocess that includes depositing a film over the first contact layer.The deposited film has a first chemical composition that includes atleast one first chemical element and a second chemical element. Aresidual chemical reaction is induced in the deposited film using asource containing the second chemical element to thereby increase thecontent of the second chemical element in the deposited film so that thedeposited film has a second chemical composition.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a process for producing a thin-film solar cell.

FIG. 2 shows a substrate configuration of a thin-film solar cell

FIG. 3 shows a superstrate configuration of a thin-film solar cell

FIG. 4 shows a multi-stage formation of a binary compound thin-film.

FIG. 5 shows a multi-stage formation of a ternary compound thin-film.

FIG. 6 shows a multi-stage formation of a CIGS compound thin-film.

FIG. 7 shows a process for depositing a CIGS* film.

FIG. 8. shows a selenization process of a CIGS* film.

DETAILED DESCRIPTION

In the following detailed description, numerous specific details are setforth in order to provide a thorough understanding of exemplaryembodiments or other examples described herein. However, it will beunderstood that these embodiments and examples may be practiced withoutthe specific details. In other instances, well-known methods,procedures, components and circuits have not been described in detail,so as not to obscure the following description. Further, the embodimentsdisclosed are for exemplary purposes only and other embodiments may beemployed in lieu of, or in combination with, the embodiments disclosed.

In accordance with the present invention, FIG. 1 illustrates an exampleof a process for manufacturing the thin-film photovoltaic (PV) device200 shown in FIG. 2. This process includes several steps starting withthe substrate preparation. Substrate 210 may be a glass sheet, a metalfoil or a plastic film. First contact layer 220 may be deposited ontosubstrate 210 using a sputtering process. The first contact may serve asthe back contact and contain metals, such as molybdenum (Mo), ortransparent conducting oxides (TCO), such as tin oxide (SnO₂). The nextoptional processing step may include a laser scribing process, in whichthe first contact layer is divided into several electricallydisconnected regions that in turn define individual PV cells.Subsequently, a thin absorber layer 230 is deposited on top of the firstcontact layer 220 using a multi-stage deposition process, which will bedescribed in detail below. The absorber material may be based on CIGS,CdTe, a-Si, organic semiconductors and other thin-film semiconductors. Athin layer of another semiconductor material may be then deposited by asputtering process, for instance, to produce a window layer 240, thusforming a pn junction with the absorber layer 230 underneath. Additionaloptional processing may include a mechanical scribing process, in whichthe absorber and window layers are divided into several electricallydisconnected regions that in turn define individual PV cells. Subsequentto these steps, a second contact layer 250 is produced to create afunctioning PV device 200. The second contact may serve as the frontcontact and contain TCO, such as indium tin oxide (ITO) or Al-doped zincoxide (AZO). In addition, this manufacturing process may include varioussubsequent steps such as additional mechanical scribing, deposition ofmetal grid contacts, anti-reflection coating, lamination, packaging,etc.

In one alternative implementation, the manufacturing process of FIG. 1may be modified to produce the thin-film PV device 300 illustrated inFIG. 3. In the modified process the window layer deposition precedes theabsorber layer deposition. In this case, PV device 300 includessubstrate 310, first contact layer 320, window layer 330, absorber layer340, and second contact 350. The first contact may serve as the frontcontact and the second contact may serve as the back contact.

As previously mentioned, absorber layers 230 and 340 may be formed usinga multi-stage process, an example of which is shown in FIG. 4. First, afilm is produced with a chemical composition AB_(1-z), which may besimilar or close to the chemical composition of a compound PVsemiconductor AB, where A and B are different chemical elements and Amay also be an alloy of different chemical elements. The value of (1−z),where z is the chemical deficiency of element B, can be defined as theratio between the initial element B content to the final element Bcontent; z may be in the range of 0.05 to 0.95, preferably in the rangeof 0.15-0.4.

It is generally preferable for the compound semiconductor AB to besufficiently stoichiometric and thus have a single phase crystallinestructure at room temperature. On the other hand, the compound materialAB_(1-z) may be non-stoichiometric and thus contain differentcrystalline phases. In the latter case the composition AB_(1-z) refersto the composition averaged across the volume of the deposited thinfilm. Compound material AB_(1-z) may also differ from the compoundsemiconductor AB in its physical properties, e.g. it may be anelectrical conductor rather than a semiconductor.

Compound material AB_(1-z) in the film may be a mixture of itsconstituent elements, e.g. a mixture of fine powders of elements A andB, respectively, wherein the characteristic particle size may be lessthan one micron, preferably in the range of 5-50 nm. It is preferablehowever that the compound material AB_(1-z) is a fully reacted,chemically stable material that is not just a mixture of its constituentelements. In this material chemical bonds are formed between itsdifferent elements and its chemical composition and crystallinestructure may be substantially homogeneous from large to very smallscales (˜few nm).

In the first step shown in FIG. 4 a thin film comprising semiconductorAB_(1-z) may be deposited using a variety of approaches, including aphysical vapor deposition (PVD) process, a magnetron sputtering process,an elemental co-evaporation, a non-vacuum nanoparticle deposition andothers. For example, a pre-reacted bulk material having chemicalcomposition AB_(1-z) may be dispersed and deposited onto a substrate inthe form of a thin film with substantially the same chemical compositionAB_(1-z). In the second step, a residual chemical reaction process isinduced in the film when it is placed in an atmosphere of vaporcontaining element B. As result of the chemical reaction, a PV absorberfilm is produced with the chemical composition AB. Chemical elements Aand B may be, for example, chemical elements selected from groups II andVI, respectively. Group II elements include such elements as Mg, Zn, Cd,and Hg, whereas group VI elements include S, Se and Te. The latter groupof elements is commonly known as chalcogens and the compound materialsbased on these elements are called chalcogenides. In one example thecompound chalcogen semiconductor AB may be CdTe.

This aforementioned process is also applicable to compound semiconductormaterials comprising more than two constituent chemical elements. Forexample, in the case of a ternary semiconductor film a film havingchemical composition ABC₁—, is first deposited onto a substrate.Afterwards, a residual chemical reaction is used to change the chemicalcomposition of the film to ABC. Chemical elements A, B and C may be, forexample, chemical elements from groups I, III and VI, respectively.Group I elements include such elements as Cu, Ag and Au, group IIIelements include Al, In, Ga and Tl, whereas group VI elements include S,Se and Te. In this case the compound semiconductor ABC may be, forexample, CuInSe₂. In addition, chemical elements from other groups maybe used, e.g. Sn from group IV.

Similarly, the multi-stage process described above may be used withquaternary semiconductors and other materials comprising more than fourelements. For instance, in the above example of the ternary compoundsemiconductor CuInSe₂, chemical elements from either group I, group IIIor group VI may be partially substituted with different elements fromthe same group. For example, Cu may be partially substituted with Ag, Inmay be partially substituted with Ga, and Se may be partiallysubstituted with S. Such substitution does not alter the basiccrystalline structure of the material and therefore does not produce anyassociated crystalline defects. It is frequently used to changeelectronic and optical properties of the material, e.g. to increase theoptical bandgap in CuInSe₂ one may substitute In fully or partially withGa.

The multi-stage formation process is particularly applicable to compoundsemiconductor materials comprising more than two constituent chemicalelements. For the formation of compound materials having multipleconstituent elements with compositional deficiencies, the multi-stageprocess may include multiple residual reaction steps each correspondingto a specific deficient element. For example, in the case of a ternarycompound material film a film having an initial chemical compositionAB_(1-y)C_(1-z) is first deposited onto a substrate, where elements Band C are deficient. Subsequently, several residual chemical reactionsteps are used to change the chemical composition of the deposited filmto ABC as shown in FIG. 5. In the first residual reaction step, anadditional quantity y of element B is provided to produce a film with anintermediate composition ABC_(1-z). In the second residual reactionstep, an additional quantity z of element C is provided to produce afilm with the final composition ABC. It also may be possible to combinemultiple residual reactions in a single processing step, where more thana single element is added to the chemical composition (e.g. B and C inthe above example).

FIG. 6 shows an example of a multi-stage formation process that employsa CIGS film. In the first step, a thin CIGS* film is produced with achemical composition Cu_(x)In_(1-y)Ga_(y)Se_(2(1-z)), which is close tothe chemical composition of a compound PV semiconductorCu_(x)In_(1-y)Ga_(y)Se₂. The Cu content x may be in the range of 0.6 to1.0, and preferably in the range of 0.8-0.95. The Ga content y, whichdetermines the CIGS optical bandgap, may be in the range of 0.1-1.0, andpreferably in the range of 0.2-0.35. The Se deficiency z may be in therange of 0.1-0.9, and preferably in the range of 0.5-0.85. In the secondstep. A residual selenization process is used to induce additionalchemical reactions in the deposited film to thereby adjust the chemicalcomposition of the film so that it has the compositionCu_(x)In_(1-y)Ga_(y)Se₂.

FIG. 7 shows schematically a method for the deposition of CIGS* thinfilms. The CIGS* film may be deposited by evaporation and/or sputteringfrom one or more targets (721, 722, 723 and 724). Target materials mayinclude high purity single elements, such as Cu, In, Ga, Se and S,partially reacted compounds and allows, such as Cu₂Se, InGa, In_(x)Se,Ga_(y)Se, CuInGa and others, and fully reacted compounds, such asCu_(x)In_(1-y)Ga_(y)Se_(2(1-z)). In this deposition process a flat rigidsubstrate 710 may be used that has been prepared and processed prior tothe deposition as described above. Alternatively, in a roll-to-rolldeposition system a flexible substrate may be used. A vacuum depositionsystem 720 may employ, for example, a magnetron sputtering tool todeposit a thin CIGS* film 730 on the substrate. DC magnetron sputteringmay be used for electrically conductive targets such as metallic alloys,which is a faster process compared to the RF sputtering that is normallyused with dielectrics. Alternatively, non-vacuum deposition methods suchas ink jet printing may be used. For example, the CIGS* bulk materialmay be pulverized into small particles, dispersed, coated on thesubstrate and then annealed to form a thin CIGS* film. These approachesof course may be utilized with other compound materials.

FIG. 8 shows schematically a method for the residual selenization step,in which a thin CIGS* film on a substrate 810 having the initialcomposition Cu_(x)In_(1-y)Ga_(y)Se_(2(1-z)) is first transported into achamber 820. Se-containing gas, such as H₂Se or Se₂, is delivered tochamber 820 and the film on the substrate 810 is heated. Under theseconditions, Se atoms diffuse into the film to complete the formation ofthe CIGS film 830, which has the final compositionCu_(x)In_(1-y)Ga_(y)Se₂. The chemical reaction may be controlled bytemperature and time. For example, residual selenization may beaccomplished using temperatures in the range from 400° C. to 550° C. anda reaction time in a range of 5 to 30 minutes. The reaction time may bechosen to allow a sufficient amount of Se (in proportion to Sedeficiency) to diffuse and react with the rest of the elements in thefilm. Selenization and other similar reactions, such as for examplesulfurization, under appropriate conditions of temperature and pressuremay be self-limiting. This implies that upon the formation of astoichiometric compound of Cu_(x)In_(1-y)Ga_(y)Se₂, furtherincorporation of Se into the film stops and the selenization reactionterminates. An example of appropriate conditions for such aself-limiting reaction may be a sufficiently high processingtemperature, at which chalcogens, which are in excess to thestoichiometry and not chemically bound with other film constituents, canexhibit a significant partial vapor pressure and may preferentially belost from the film through vaporization. This is particularly convenientin large scale manufacturing, where processes requiring less control areless expensive, more reliable and thus more attractive. After residualselenization, the CIGS film is fully reacted, forming a chalcopyritepolycrystalline structure with a minimal amount of crystalline defectsassociated with deficiencies of its constituent elements (Se inparticular).

In accordance with the present invention, the multi-stage thin filmdeposition process may be applied to a variety of other compoundmaterials. Examples include:

-   -   1. Cu_(x)In_(1-y)Ga_(y)S₂. First, a thin film is produced in a        PVD process with a chemical composition        Cu_(x)In_(1-y)Ga_(y)S_(2(1-z)). Cu content x may be in the range        of 0.6 to 1.0, preferably in the range of 0.8-0.95. Ga content y        may be in the range of 0.05-1.0, preferably in the range of        0.1-0.35. S deficiency z may be in the range of 0.1-0.9,        preferably in the range of 0.5-0.85. Second, a residual        sulferization process is used to adjust the chemical composition        of the film to Cu_(x)In_(1-y)Ga_(y)S₂.    -   2. Cu_(x)In_(1-y)Ga_(y)S_(2z)Se_(2(1-z1)). First, a thin film is        produced in a PVD process with a chemical composition        Cu_(x)In_(1-y)Ga_(y)S_(2z)Se_(2(1-z1)). Cu content x may be in        the range of 0.6 to 1.0, preferably in the range of 0.8-0.95. Ga        content y may be in the range of 0.05-1.0, preferably in the        range of 0.1-0.3. S content z may be in the range of 0.0-1.0,        whereas Se deficiency z1 may be in the range of 0.1-1.0 so that        z1>z. Second, a residual selenization process is used to adjust        the chemical composition of the film to        Cu_(x)In_(1-y)Ga_(y)S_(2z)Se_(2(1-z)). For this compound either        residual selenization, sulfurization or a combination of the two        processes (e.g. employing vapors of both S₂ and Se₂) may be used        in the last stages of the deposition process.    -   3. Cu_(x)In_(1-y)Al_(y)Se_(2(1-z)). First, a thin film is        produced in a PVD process with a chemical composition        Cu_(x)In_(1-y)Al_(y)Se_(2(1-z)). Cu content x may be in the        range of 0.6 to 1.0, preferably in the range of 0.8-0.95. Al        content y may be in the range of 0.05-1.0, preferably in the        range of 0.4-0.6. Se deficiency z may be in the range of        0.1-0.9, preferably in the range of 0.5-0.85. Second, a residual        selenization process is used to adjust the chemical composition        of the film to Cu_(x)In_(1-y)Al_(y)Se₂.    -   4. Cu_(x1)Ag_(x2)In_(1-y)Ga_(y)Se₂. First, a thin film is        produced in a PVD process with a chemical composition        Cu_(x1)Ag_(x2)In_(1-y)Ga_(y)Se_(2(1-z)). Cu content x1 and Ag        content x2 may be in the range of 0.1 to 1.0, so that x1+x2<1.        Ga content y may be in the range of 0.05-1.0, preferably in the        range of 0.1-0.35. Se deficiency z may be in the range of        0.1-0.9, preferably in the range of 0.5-0.85. Second, a residual        selenization process is used to adjust the chemical composition        of the film to Cu_(x1)Ag_(x2)In_(1-y)Ga_(y)Se₂.    -   5. Cu_(x)Zn_(1-y)Sn_(y)S₂. First, a thin film is produced in a        PVD process with a chemical composition        Cu_(x)Zn_(1-y)Sn_(y)S_(2(1-z)). Cu content x may be in the range        of 0.6 to 1.0, preferably in the range of 0.8-0.9. Sn content y        may be in the range of 0.05-1.0, preferably in the range of        0.3-0.45. S deficiency z may be in the range of 0.1-0.9,        preferably in the range of 0.5-0.85. Second, a residual        sulferization process is used to adjust the chemical composition        of the film to Cu_(x)Zn_(1-y)Sn_(y)S₂.    -   6. Cu_(x)InS₂. First, a thin film is produced in a PVD process        with a chemical composition Cu_(x)InS_(2(1-z)). Cu content x may        be in the range of 0.6 to 1.0, preferably in the range of        0.8-0.9. S deficiency z may be in the range of 0.1-0.9,        preferably in the range of 0.5-0.85. Second, a residual        sulferization process is used to adjust the chemical composition        of the film to Cu_(x)InS₂.    -   7. CdTe. First, a thin film is produced in a PVD process with a        chemical composition CdTe_(1-z). Te deficiency z may be in the        range of 0.1-0.9, preferably in the range of 0.5-0.85. Second, a        residual chemical process with Te vapor is used to adjust the        chemical composition of the film to CdTe.    -   8. Cd_(1-x)Mg_(x)Te. First, a thin film is produced in a PVD        process with a chemical composition Cd_(1-x)Mg_(x)Te_(1-z). Mg        content x may be in the range of 0.0-1.0, preferably in the        range of 0.1-0.6. Te deficiency z may be in the range of        0.1-0.9, preferably in the range of 0.5-0.85. Second, a residual        chemical process with Te vapor is used to adjust the chemical        composition of the film to Cd_(1-x)Mg_(x)Te.    -   9. Cd_(1-x)Zn_(x)Te. First, a thin film is produced in a PVD        process with a chemical composition Cd_(1-x)Zn_(x)Te_(1-z). Zn        content x may be in the range of 0.0-1.0, preferably in the        range of 0.1-0.6. Te deficiency z may be in the range of        0.1-0.9, preferably in the range of 0.5-0.85. Second, a residual        chemical process with Te vapor is used to adjust the chemical        composition of the film to Cd_(1-x)Zn_(x)Te.

These and other suitable compound materials may also include minordopants that have relatively small concentrations and thus do notsignificantly alter the overall chemical composition and the associatedcrystalline structure. For example, CIGS compounds may include 0.01-0.3at. % of sodium (Na), which facilitates grain growth in CIGS films andalso improves their electrical properties. In this case Na may be addedto the bulk material and then transferred to the film with other CIGSelements during deposition. Also, in the initial deposition of athin-film its constituents may include temporary chemical elements thatmay be removed from the film during subsequent processing steps. Forexample, such a deposition process may involve one of the oxidecompounds, i.e. CuO, In₂O₃ and Ga₂O₃, where oxygen may be later removedby chemical reduction using hydrogen gas.

The multi-stage thin film formation process provides several advantagesover the existing film forming processes. Residual chemical reactions inthe deposited film enable more flexible control of the film'smorphology. Specifically, they promote polycrystalline grain growth andgrain fusing. The volumetric expansion of the film associated with thesechemical reactions removes pinholes and improves intergrain electricaltransport. It is also known that the interlayer adhesion may becompromised by the film's exposure to the high temperatures typicallyrequired by chemical reactions in the existing deposition processes. Themulti-stage process can improve the adhesion between the absorber layerand the first contact layer by substantially reducing the amount ofchemical reactions required in the film after deposition.

Example 1

In this example the multi-stage process is used to deposit a CIGS filmwith the approximate chemical composition of CuIn_(0.7)Ga_(0.3)Se₂ sothat it may be used in a CIGS solar cell. As a first step, powders ofCu, In, and Ga Se are mixed in the proportions corresponding toCu_(0.9)In_(0.7)Ga_(0.3). The mixture is melted and fully alloyed in apressurized enclosed vessel at a temperature of about 1000° C. In thiscase Cu deficiency is a desired characteristic feature of the CIGS filmwhich is known to improve electrical properties of CIGS films. Theresulting bulk material is then used to produce a target suitable foruse in a DC sputtering system. This target and a pure Se target is thenused in a magnetron co-sputtering system to deposit a thin CIGS* filmwith a chemical composition of Cu_(0.9)In_(0.7)Ga_(0.3)Se_(1.6). A sodalime glass coated with a 0.7 □m thick Mo layer is used as a substrate.The substrate temperature during the deposition may be in the range of100° C. to 550° C. Lower substrate temperature may reduce Se mobilityand enable better compositional control of the film. The deposited CIGS*film thickness may be in the range of 1 to 3 □m. In the last step, thisfilm is heated in the presence of H₂Se for 10-30 min at a temperature ofabout 400-500° C. During this process additional Se is incorporated intothe film so that at the end of the process the film composition is aboutCu_(0.9)In_(0.7)Ga_(0.3)Se₂.

Example 2

In this example the multi-stage process is used to deposit a CIGS filmwith the approximate chemical composition of CuGaSe₂ so that it may beused in a CIGS solar cell. As a first step, Cu, Ga and Se areco-evaporated to produce a film with an average compositioncorresponding to Cu_(0.9)GaSe_(1.7). A polyimide film coated with a˜0.5-1 □m thick Mo layer is used as a substrate. The deposited CIGS*film thickness is in the range of 0.5 to 2 □m. In a third step, thisfilm is heated in the presence of H₂Se for 10-30 min at a temperature ofabout 400-500° C. During this process additional Se is incorporated intothe film so that at the end of the process the film composition is aboutCu_(0.9)GaSe₂.

1. A method for producing a film of compound material comprising thesteps of providing a substrate; depositing a film on the substrate,wherein the deposited film has a first chemical composition thatincludes at least one first chemical element and at least one secondchemical element; inducing at least one residual chemical reaction inthe deposited film using a source containing at least one secondchemical element to thereby increase the content of at least one secondchemical element in the deposited film so that the deposited film has asecond chemical composition; wherein the content of at least one secondelement in the second chemical composition is larger than the content ofat least one second element in the first chemical composition.
 2. Amethod of claim 1 wherein said at least one first chemical elementcomprises a plurality of chemical elements.
 3. A method of claim 1wherein said at least one second chemical element comprises a pluralityof chemical elements.
 4. A method of claim 1 wherein said at least onefirst chemical element comprises at least one element from group I andgroup III chemical elements.
 5. A method of claim 1 wherein said atleast one second chemical element comprises an element from group VI. 6.A method of claim 1 wherein said substrate comprises a first contactlayer.
 7. A method of claim 1 wherein at least one second elementcontent in the deposited film is increased by at least 10%.
 8. A methodof claim 1 wherein said step of depositing the film comprises the stepsof preparing the compound bulk materials from said at least one firstchemical element and at least one second chemical element, forming atleast one sputtering target from the compound bulk materials andsputtering the compound bulk materials onto the substrate.
 9. A methodof claim 1 wherein said step of depositing the film comprises the stepof evaporating from bulk materials comprising at least one first and atleast one second chemical element onto the substrate.
 10. A method ofclaim 1 wherein said at least one residual chemical reaction is producedat a temperature of at least 400° C.
 11. A method of claim 1 whereinsaid the step of inducing at least one residual chemical reactionincludes exposing the film to a vapor that includes at least one secondchemical element.
 12. A method of claim 1 wherein said second chemicalcomposition is stoichiometric.
 13. A method of claim 1 wherein said atleast one residual chemical reactions produce a film having a singlecrystalline phase.
 14. A method of claim 1 wherein said first chemicalcomposition comprises Cu, In, Ga and Se.
 15. A method of claim 1 whereinsaid first chemical composition comprises Cd and Te.
 16. A thin-filmformed in accordance with the method of claim
 1. 17. A method forproducing a thin-film solar cell comprising the steps of depositing afirst contact layer on a substrate; producing an absorber layer over thefirst contact layer; depositing a window layer and a second contactlayer over the absorber layer, wherein the absorber layer is produced bya process that includes: depositing a film over the first contact layer,wherein the deposited film has a first chemical composition thatincludes at least one first chemical element and a second chemicalelement; inducing a residual chemical reaction in the deposited filmusing a source containing the second chemical element to therebyincrease the content of the second chemical element in the depositedfilm so that the deposited film has a second chemical composition.
 18. Athin-film solar cell formed in accordance with the method of claim 17.